About the Brinell hardness test

The Brinell hardness test was originally developed in the late 1800s by the Swedish engineer of the same name. He wanted to find a method to control the quality/hardness of steel. His solution was to press a railway wheel-bearing ball into the material and then measure the size of the mark it left. The method proved reliable and in 1900 the Brinell hardness test was officially born.

Today, the Brinell test is performed using a Brinell hardness test unit. The instrument presses a tungsten carbide ball into the sample with a known force, and then optically measures the diameter of the impression, which than is calculated into the Brinell hardness value (HBW).

• Indenter sizes: 1, 2.5, 5 and 10 mm
• Loads: From 1 kgf to 3000 kgf
• Maximum hardness: 650 HBW.

A hardness test for larger samples
As the Brinell hardness test (HBW) indentation leaves a relatively large impression, the Brinell hardness test is better suited to larger samples with a coarse or inhomogeneous grain structure, such as castings and forgings.

Good to know
HBW stands for Hardness Brinell Wolfram carbide. Wolfram carbide (= tungsten carbide) underlines that newer Brinell standards call for the use of tungsten carbide balls, as opposed to the (softer) steel balls previously used (HBS). Values will differ at higher hardness.

The Brinell Hardness calculation​

The Brinell Hardness is calculated by optically measuring the length of the longest diagonal of the impression left by the diamond indenter. The measured indentation is then converted into a hardness value using a standardized formula or reference table. Modern hardness testers automatically calculate and display the HK value through integrated software.

HBW = constant × Test force / idealized surface area of indentation
HBW =  0.102 × 2F / (π D² (1-√(1-(d² /D²)))​

HBW: Brinell hardness with Tungsten Carbide (Wolfram)​
F: Test load (kgf)​
D: Length of the long diagonal (mm)​
d: Mean diameter of the indentation

Brinell Loads

• Indentation time: 10-15 seconds
• Sample thickness ASTM: At least 10 times the indentation depth
• Sample thickness ISO: At least 8 times the indentation depth.

Download our Application note about Hardness Testing

Indent spacing​

A Brinell indentation not only creates a visible deformation, it also changes the structure around the indent crater. To eliminate the influence of neighboring tests and ensure reliable measurements, the standards prescribe specific spacing between indentations.

Edge spacing: ≥ 2.5 × the mean indentation diagonal, measured from the center of the indentation to the nearest specimen edge.
Indent-to-indent spacing: ≥ 3 × the mean indentation diagonal, measured between the centers of adjacent indentations.​

Indentation diameter​

The indentation diameter should be measured in two perpendicular directions, with the average value used for hardness calculation. For ground surfaces, measurements at approximately 45° to the grinding direction are recommended.
Automatic measurement systems (camera) may apply alternative averaging algorithms.

Prepare for a Brinell hardness test​

Specimen​
The test surface shall be smooth, even, and free from oxide scale, contaminants, and lubricants. The applicable standards specify that the surface finish must permit accurate measurement of the indentation diameter, meaning the surface finish must allow the indentation edges to be clearly identified.

When using small ball indenters (1 mm and 2.5 mm), fine grinding or polishing may be necessary to obtain a suitable surface finish. Care should be taken during preparation to avoid introducing heat that could alter the specimen's microstructure and influence the hardness result.

Equipment​
To ensure reliable and traceable hardness results, the Brinell instrument should undergo direct verification and calibration once a year by an accredited calibration laboratory. In-between direct calibrations, operators are encouraged to perform regular indirect verifications using certified reference test blocks with known Brinell values.​

Environment​
Thanks to its relatively high test forces and large indentation size, the Brinell method is inherently robust and well suited for both laboratory and workshop environments. However, because the hardness value is determined by optical measurement of the indentation, a reasonably clean, dust-free environment is recommended to ensure reliable operation and minimize maintenance and service requirements. For accurate and repeatable results, the hardness tester should be installed on a stable, rigid workbench or table. Vibrations from nearby machinery, foot traffic, or other external sources should be minimized or eliminated whenever possible.
ISO and ASTM standards specify an operating temperature between 10 °C and 35 °C.​

Operator​
The operator is responsible for ensuring that the specimen is correctly positioned and remains stable throughout the complete test cycle. To prevent measurement errors caused by material deformation, indentations should be spaced sufficiently apart and away from the specimen edges.

For specific applications, always consult the relevant Brinell testing standard to confirm the required spacing and testing conditions.

Brinell Methods and Application

There are a number of common Brinell hardness test methods, with corresponding materials and hardness ranges. Most test methods can be performed on any Brinell hardness testing machine.

The Brinell methods are generally divided into four subgroups (HB30, HB10, HB5, HB2.5), each suitable for a different group of materials.

• Each subgroup has the same force/diameter ratio (F/D2)
• Measured Brinell hardnesses can only be compared within individual subgroups.

* Materials mentioned in the table are examples of typical materials only.

Explanation

• HBW 2.5/187.5: Brinell 2.5 mm tungsten carbide ball and 187.5 kgf load.
• HBW 5/750: Brinell 5.0 mm tungsten carbide ball and 750 kgf load.

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